1. Field of the Invention
This invention relates to a protocol and bit rate independent test system for digital communication systems. More particularly, this invention relates to systems that are able to detect bit errors on a digital communications channel regardless of format or rate.
2. Description of the Background Art
In digital communication systems, particularly dense wavelength division multiplexed (DWDM) optical systems, multiple signal formats may exist on the same communications link at different wavelengths. It is useful to be able to extract a signal and determine its health. Hence, there is a need to have a protocol independent and bit rate independent test capability that can (1) detect a single defective bit, (2) monitor the traffic error rate and (3) provide a protection switch signal within milliseconds of exceeding the error threshold condition, and to embed such a test head on every wavelength of the communications system, such as a DWDM system, so as to support such systems where the traffic on different wavelength channels are different protocols and different bit rates. The channels will contain unknown protocols as purchased by the customers. In such a scenario, the customer can buy a wavelength of light and the service carrier does not control the rate or protocol of the traffic. This creates a difficult testing condition since the service provider needs a way to verify error free transmission, provide protection switching and location of the fault condition. The service provider therefore needs a test capability that is bit rate and protocol independent.
Heretofore it has been known that the performance of a communications link, such as a fiber optic link, known as Q factor measurement, is reflected by interpreting the eye pattern of the waveforms of a series of pulses. A typical eye pattern is depicted in FIG. 1. The optimal time for sampling the data signals and the optimal level (the threshold level) at which to distinguish between zeros and ones is the center of the eye. Further, it is known that the height of the central eye opening determines noise margin in receiver output, the width of the signal band at the corner of the eye depicts the jitter or variation in pulse timing in the system, the thickness of the signal line at top and bottom of the eye is proportional to noise and distortion in the receiver output, and transitions between top and bottom of the eye pattern show the rise and fall times of the signal that can be measured on the eye pattern. More complete descriptions of eye measurements (and of receivers and other components) can be found in the following references, the disclosures of each of which are incorporated by reference herein: Joseph C. Palais, Third Edition Fiber Optic Communications, Prentice Hall, Englewood Cliffs, N.J., 1992; John B. Anderson, Digital Transmission Engineering, IEEE Press, Piscataway, N.J., 1999; Stephen B. Alexander Optical Communication Receiver Design, SPIE Optical Engineering Press, Bellingham, Wash., 1997; Govind P. Agrawal Fiber-Optic Communication Systems, John Wiley and Sons, Inc, New York N.Y., 1997; Kaminow and Koch, Optical Fiber Telecommunication IIIA, Academic Press Limited, 1997; and Anderson and Lyle, Technique for Evaluating System Performance Using Q in Numerical Simulations Exhibiting Intersymbol Interference, Electronics Letters, Vol. 30, No. 1, Jan. 6, 1994. Unfortunately, Q measurements does not provide a real-time measurement or a single-bit error detection capability.
An object of this invention is to provide a bit rate and protocol independent test apparatus and method that can be embedded on every wavelength of a communications system such as a DWDM system.
Another object of this invention is to provide a bit rate and protocol independent capability that can (1) detect a single defective bit, (2) monitor the traffic error rate and (3) provide a protection switch signal within milliseconds of exceeding the error threshold condition of the communications system.
Another object of this invention is to provide an apparatus and method for conducting eye measurements to determine the Q factor.
The foregoing has outlined some of the pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.
For the purpose of summarizing this invention, this invention preferably comprises a discrete implementation or custom Application Specific Integrated Circuit (ASIC) including the major components of a receiver, variable rate CRU (Clock Recovery Unit), and a very high speed threshold sampling module for various static or dynamic sampling points positioned in an array (two-dimensional or linear) that is able to instantaneously determine the shape of the eye of a digital communication system, and very high speed logic to process the data. This basic system meets the requirement and is the basis for a universal test set. In addition to bit error detection, it measures the frequency of the signal. Further enhancements includes creating oscilloscope eye diagrams and bit capture and post processing to identify the signal protocol by post processing. The preferred embodiment of the invention has particular application in optical communications systems, such as DWDM optical systems, employing an optical receiver; however, the invention may be incorporated into various other types of digital communication systems employing various other types of receivers without departing from the spirit and scope of this invention.
The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifing or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.